1. Field of the Invention
The present invention relates to high performance pneumatic radial tires, particularly to high performance pneumatic radial tires having uneven wear resistance greatly improved in use under the application of a large lateral acceleration without lowering drainage performance on wet roads.
2. Related Art Statement
As shown in a cross sectional view of FIG. 8, in a conventional pneumatic radial tire provided with a plurality of circumferential grooves extending zigzag or straight in the circumferential direction of the tire, it is a common practice that opposed groove walls 34 of each of the circumferential grooves 33 are spaced from each other toward the surface 31 of a tread such that angles .alpha. defined between normals 32 erected perpendicularly to the tread surface 31 and extensions of the opposed groove walls 34 may be equal to each other. Also, the groove walls 34 are connected together with a bottom wall 35 constituted by a curved surface 35 having a specific radius of curvature R.
However, in the conventional tire with the circumferential grooves having the above shape during, when a large lateral acceleration is applied to the circumferential groove during high speed running on a circuit or the like, the outer portion of the tire particularly positioned on the outer side of turning is deformed as shown in a sectional view of FIG. 9 owing to a collapse of the circumferential groove 33. Thus, that portion of the tread which comes to particularly firmly contact the ground owing to this deformation. That is, a shoulder portion A adjoining the treading surface of the tread and a land edge portion B adjoining the inside of the outermost circumferential groove 33 in the width direction of the tire bear great reaction forces from the road surface. As a result, as shown in FIG. 10, there is a problem in that both the portions A and B are abnormally worn at a relatively early state.
Under the circumferences, in order to solve the above problem, a trial has been made to improve a wearing profile by narrowing the width of the circumferential grooves to such a degree that the opposite groove walls may contact each other and consequently the deformation of the tire may be restrained in the state that the opposed groove walls contact each other. However, according to this technique, since the volume of the grooves of the tire becomes smaller, the drainage performance on the wet road inevitably deteriorates.
On the other hand, Japanese Patent Application Laid open No. 60-193,704 describes, a pneumatic tire having improved drainage performance, cornering stability and gripping property during turning improved. The tire has a cross angle of that entire groove wall relative to a normal line to the surface of a tread which is located outside in the axial direction of the tire between the opposed groove walls of each of circumferential grooves which is made greater than that of the other located inside in the tire axial direction. The cross angle between each of the axially outer groove walls and the normal is set in a range of 10.degree.-30.degree.. In this tire, since rigidity of the land portions defined by the outer groove walls increases, the above-mentioned uneven wear can be suppressed to some degree. However, since the outer wall of the circumferential groove is merely inclined from the bottom of the groove toward the surface of the tread to gradually increase the width of the groove, the circumferential groove is deformed in substantially the same manner as shown in FIG. 9 when a particularly large lateral acceleration is applied to the tire. Consequently, occurrence of abnormal wearing as shown in FIG. 10 cannot be prevented to a satisfactory degree.
Japanese Patent Application Laid-open No. 58-128,905 describes a tire in which walls of wavy or zigzag circumferential grooves are positioned on a side from which the tire is extracted from a mold are provided with groove-enlarging, inclined surface adjoining the treating surface, so that releasing of the tire from the mold may be facilitated in a full mold production process. According to this tire, the uneven worn state is even slightly improved. However, in this tire, since the circumferential groove positioned near the edge of the tread has a stepped portion on the groove wall having the above-mentioned groove-enlarging, inclined surface at a radially inner location, the groove wall of the circumferential groove is bent around the axially outer edge of the stepped portion when the tire receives great reaction forces from the road surface under application of a great lateral acceleration. As a result, the tire contacts the ground in a similar state shown in FIG. 9, and finally the tire is worn in the substantially same state shown in FIG. 10.
Further, Japanese Utility Model Registration Application Laid-open No. 63-72,024 describes a tire in which a chamfer continuing in the circumferential direction of the tire is provided in a contact between one of groove walls of a circumferentially annular groove and the surface of a tread. The inner edge of the chamfer as viewed in the radial direction of the tire is inwardly located in a range of 3-15% of the annular groove from the surface of the tread so that the tire may easily be released from a mold. However, although the worn state can only slightly be improved in this tire, the deformation as shown in FIG. 9 and the uneven wear as shown in FIG. 10 cannot effectively be prevented because the width of the cut is too small.